1. Field of the Invention
The invention generally pertains to digital data communication systems. More particularly, the invention relates to methods and apparatus which use bit mapping and frame alignment techniques to (1) locate and extract sub-rate circuit data originially placed into a high speed serial bit stream in accordance with a first predefined sub-rate circuit protocol, and (2) create a reformatted high speed serial data output stream, with the extracted sub-rate circuit data inserted therein in accordance with a second predefined sub-rate circuit protocol. The methods and apparatus set forth are particularly useful in converting between a single stage time division multiplexed data format and a two stage time division multiplexed format, e.g., the CCITT X.50 and I.463 protocols.
2. Description of the Prior Art
State of the art digital data communication switching systems are typified by the systems described in copending patent application Ser. No. 07/103,611, filed Oct. 1, 1987, entitled "High Speed Communication Processing System", and Ser. No. 07/103,612, also filed Oct. 1, 1987, entitled "A Digital Data Communications System". Both of these applications, hereby incorporated by reference, are assigned to the same assignee as this invention.
The systems taught in these applications are defined as having m slots of data per frame and n fragments per slot to yield a total of m.times.n fragments per frame.
For example, the invention taught in the No. 07/103,612 application permits a T1 line to be divided up into 24 slots, each 64 Kbps wide, with 8 fragments being defined per slot (each representing 8 Kbps bandwidth). This yields 192 fragments with an 8 Kbps signalling channel left on the T1 line. Bandwidth allocation is also contemplated for 2.048 Mbps Inter Module Links ("IMLs") within a given node, where 32 slots, each 64 Kbps wide, can be defined (each again with eight 8 Kbps fragments).
Also taught in the referenced applications are means for allocating and deallocating bandwidth on the communication lines of the system using bit maps, how to perform allocation in a manner which minimizes call blocking, contention, etc., and how to interconnect Customer Premise Equipment (CPE) to the node oriented network via User Interfaces (UI), Network Processors (NPs), Switch Matrices (SMs), Network Interfaces (NIs), etc., via the IMLs in a given node.
The systems taught in the referenced applications support the CCITT I.463 standard for multiplexing of sub-rate channels. Also, these systems have the capability of switching fragment vs. slot packets, i.e., have an improved switching granularity as compared with the 64 Kbps switch granularity to which the I.463 standard was designed.
For the illustrative 32 slot, 8 fragments per slot IML bandwidth scheme set forth hereinbefore, I.463 mandates that sub-rate channels (defined herein as a 19.2 Kbps channel or any submultiple thereof) be assigned to 64 Kbps of bandwidth (a whole slot) before being transmitted to the switch. According to the I.463 standard, two layers of rate adaptation are performed to condition sub-rate channel data to be transmitted in the 64 Kbps packets.
Given the improved switching granularity and bandwidth allocation capabilities of state of the art systems, such as those set forth in the referenced copending patent applications, it was an object of copending patent application Ser. No. 07/308,705, filed on Feb. 9, 1989, entitled "Methods and Apparatus For Multiplexing Sub-rate Channels In A Digital Data Communication System", assigned to the same assignee as this invention, to improve the efficiency of bandwidth usage over the I.463 standard for sub-rate channel bandwidth allocation. This copending application is hereinafter referred to as the "Improved I.463 Patent Application".
It was also an object of the Improved I.463 Patent Application to be able to fully use the switching capabilities (granularity) of the communication system. Since the inventions taught in the incorporated patent applications made it possible to switch fragments of less then 64 Kbps, it was recognized that it would be desirable if sub-rate channel assignments were keyed to the granularity of the switch which in turn could be ideally matched to the choice of fragment size.
Patent application Ser. No. 07/308,705, filed Feb. 9, 1989, is hereby incorporated by reference.
In short, the Improved I.463 Patent Application, describes how to improve the efficiency of bandwidth usage over the CCITT I.463 standard, for switching systems having a granularity of better then 64 Kbps (e.g. 8 Kbps)
The efficiency of bandwidth usage over the I.463 standard is achieved (in the context of the illustrative example set out above) by implementing a new rate adaptation scheme in which sub-rate data channels are assigned to an 8 Kbps fragment, or multiples thereof. For example, a 2.4 Kbps channel is assigned to an 8 Kbps envelope, 9.6 Kbps is assigned to two 8 Kbps envelopes, etc. Multiple envelopes (each 8 Kbps wide) are then multiplexed effecting a potential 8 fold improvement over existing methods and apparatus for implementing the I.463 standard. In the illustrative context, the key is to use the 8 Kbps granularity of the switch rather then the I.463 standard of assigning sub-rate data to a whole 64 Kbps slot.
The CCITT I.463 standard (and/or improved versions thereof) are not in universal use in digital communication systems that carry sub-rate circuits. The Japanese for example, utilize the CCITT X.50 recommendations (to be reviewed in detail hereinafter) for formatting sub-rate circuit data. If wide area networks are to include nodes where different format criteria are observed, particularly regarding the placement of sub-rate data in their output bit streams, then methods and apparatus to support format conversion in such a network need to be devised to make communication between the users at these nodes possible.